Let $a, b$ be two real numbers such that $ab < 0$. If the complex number $z_1 = \dfrac{a + ib}{b + i}$ is of unit modulus and $z_2 = a + ib$ lies on the circle $|z| = |2z - 1|$, then a possible value of $\left[\dfrac{a + 1}{4b}\right]$, where $[t]$ is the greatest integer function, is: